Lipids comprise a class of hydrophobic macromolecules that include fatty acids and cholesterol. These ubiquitous molecules function as a major source of cellular energy and constitute a primary component of cell membranes. In addition, lipids function in many signaling pathways as components of hormones and other intracellular messengers. Animal cells store fatty acids in the form of triacylglycerols, which consist of three chains held in ester linkage with one molecule of glycerol. Triacylglycerols, along with cholesterol, are transported through the bloodstream in the form of lipoprotein emulsions. These emulsions contain a variety of apolipoproteins which associate with and permit lipids to be transported in an aqueous environment. Adipocytes store excess triacylglycerol in the cytoplasm as large lipid globules which may occupy up to ninety-five percent of the cell volume.
Adipocyte differentiation has been studied using a variety of cultured adipogenic cell lines that terminally differentiate into adipocytes upon reaching confluence or stimulation with an appropriate agent. These studies have led to the discovery of a 50-kDa membrane-associated protein, adipose differentiation-related protein (ADRP), that serves as an early marker for preadipocyte differentiation (Jiang, H. P. and Serrero, G. (1992) Proc. Natl. Acad. Sci. 89: 7856-7860). ADRP has also been found in milk-secreting cells where it is associated with the surface of fat droplets. A human homolog of ADRP, adipophilin, has been reported, and its protein sequence is highly conserved in mouse, rat, and cow (Heid, H. W. et al. (1996) Biochem. J. 320: 1025-1030). Thus, ADRP is suggested to be associated with fat globule formation in different cell types.
ADRP expression correlates strongly with differentiation of preadipocytes in culture and the formation of fat globules. In stimulated preadipocytes, ADRP expression begins one day earlier than other known markers of adipocyte differentiation, e.g., fatty-acid binding protein and lipoprotein lipase. ADRP mRNA expression increases 100-fold within a day after induction, and continues to increase during adipocyte differentiation. ADRP expression is stimulated by addition of accelerators of differentiation, e.g., dexamethasone, to adipogenic cells. Conversely, ADRP expression is inhibited by addition of inhibitors of differentiation, e.g., transforming growth factor beta and tumor necrosis factor (Jiang, H. P. et al. (1992) Cell Growth Differ. 3: 21-30). ADRP expression can also be induced in rat liver tissue by treatment with etomoxir, an irreversible inhibitor of carnitine palmitoyltransferase I; induced ADRP expression levels correlate with accumulation of lipid droplets in the liver (Steiner, S. et al. (1996) Biochem. Biophys. Res. Commun. 218:777-782).
The subcellular localization of ADRP in both adipocytes and milk secreting cells indicates that although ADRP lacks a predicted signal sequence or transmembrane domain, it nevertheless appears to be associated with lipid droplets in both cell types. (Jiang and Serrero, supra; Heid et al., supra). In milk secreting cells, ADRP was identified as a major constituent of milk-lipid-globule-membrane (MLGM). Another major component of MLGM, butyrophilin, binds tightly to ADRP (Heid et al., supra).
ADRP and its human homolog, adipophilin, share significant sequence homology with perilipin, a hormonally-regulated phosphoprotein that surrounds lipid storage droplets in adipocytes where it is the predominant cAMP-dependent protein kinase substrate. In differentiating preadipocytes, perilipin expression appears at the onset of triacylglycerol accumulation and transcript level increases in parallel with lipid accumulation. (Greenberg A. S. et al. (1991) Clin. Res. 39: 287A). Thus, the related proteins adipophilin, ADRP, and perilipin appear to be members of a distinct set of proteins that associate with lipid globules and function in lipid metabolism (Greenberg A. S. et al. (1993) Proc. Natl. Acad. Sci. 90: 12035-12039).
Many human diseases such as atherosclerosis, hyperlipidemia, obesity, and diabetes have been related either directly or indirectly to abnormalities in lipid metabolism. In addition, a neurodegenerative disorder, Alzheimer's disease, has been related to abnormal lipid metabolism via its genetic linkage with apolipoprotein E (apoE). Apo(E) is a component of very low density lipoproteins that plays a role in neuronal repair and remodeling (Weisgraber, K. H. and Mahley, R. W. (1996) FASEB J. 10: 1485-1494).
The discovery of a new human adipophilin-like protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of disorders of lipid metabolism, neurodegenerative disorders, and cancer.